In recent years, with entry of HDDs into AV products and home electrical products, spindle motors are required to be compact, thin, highly accurate and have long life spans. FIG. 10 shows a construction of the spindle motor. A base 51 is provided with a sleeve 52 having a bearing hole. A shaft 53 is inserted through the bearing hole. A hub 54 which receives a disk 65 and a clamp 55 for holding the disc 65 on the hub 54 are provided at an upper end portion of the shaft 53. The clamp 55 is fastened to the shaft 53 by screwing a clamp screw 56 into a screw hole 57 (female screw) formed in the shaft 53.
The shaft 53 is constructed by a body shaft part 53a, a tip end shaft part 53b formed at an upper end of the body shaft part 53a and a thrust flange part 53c formed at a lower end of the body shaft part 53a. A diameter of the tip end shaft part 53b is smaller than a diameter of the body shaft part 53a. The tip end shaft part 53b is fitted into the hub 54.
A diameter of the thrust flange part 53c is larger than the diameter of the body shaft part 53a. The hub 54 is provided with a magnet 66. A stator 67 with a coil wound around is provided at the base 51 to be opposed to an inner peripheral side of the magnet 66.
In order to increase accuracy and a life of the spindle motor, a hydrodynamic bearing with the following structure is adopted for the bearing.
Namely, a thrust plate 58 which receives a thrust load which acts on the shaft 53 is provided at a lower end portion of the sleeve 52.
A gap 60a is formed between an outer peripheral surface of the body bearing part 53a of the shaft 53 and an inner peripheral surface of the sleeve 52. A gap 60b is formed between an outer peripheral surface of the thrust flange part 53c and an inner peripheral surface of the sleeve 52. A gap 60c is formed between a top surface of the thrust flange part 53c and a lower surface of the sleeve 52 which is opposed to the top surface of the thrust flange part 53c. A gap 60d is formed between a lower surface of the thrust flange part 53c and a top surface of the thrust plate 58. Operating oil 61 which is an operating fluid is filled in each of the gaps 60a to 60d. 
A pair of radial bearing parts 62a and 62b are provided in a direction of an axis 68 (vertical direction) at the inner peripheral surface of the sleeve 52. A dynamic pressure generating groove is formed in each of the radial bearing parts 62a and 62b. 
A main thrust bearing part 63 is provided on a top surface of the thrust plate 58. A dynamic pressure generating groove is formed in the main thrust bearing part 63.
An auxiliary thrust bearing part 64 is provided on a top surface of the thrust flange part 53c. A dynamic pressure generating groove is formed in the auxiliary thrust bearing part 64.
The dynamic pressure generating groove of the main thrust bearing part 63 provided at the top surface of the thrust plate 58 may be provided on an undersurface of the thrust flange part 53c. 
In such a construction, when a coil is energized, a rotary magnetic field occurs to the stator 67, with which the hub 54 to which the magnet 66 is mounted rotates, and the disk 65 held on the hub 54 rotates. On this occasion, the shaft 53 fixed to the hub 54 rotates, dynamic pressure is generated by each of the bearing parts 62a, 62b, 63 and 64, a radial load is supported by the radial bearing parts 62a and 62b, and a thrust load is supported by the main thrust bearing part 63 and the auxiliary thrust bearing part 64.
However, in the above described conventional type, an outside diameter of the shaft 53 becomes small with miniaturization of the spindle motor, and a wall thickness t in a diameter direction between the outer peripheral surface of the body bearing part 53a of the shaft 53 and the screw hole 57 becomes small. Therefore, when the clamp screw 56 is screwed into the screw hole 57 and fastened thereto, as shown by the solid line in FIG. 11, the upper portion of the shaft 53 bulges outward (outside diameter direction) and deforms, and an upper portion of the gap 60a is reduced in the diameter direction and narrower than a lower portion. As a result, the adverse effects that the generated dynamic pressure of the radial bearing part 62a becomes large, the pressure in the vicinity of the gap 60d rises via the gaps 60a, 60b and 60c, and the shaft 53 excessively floats (excessive floatation).
FIGS. 10 and 11 show the spindle motor adopting the hydrodynamic bearing, and Japanese Patent Laid-Open No. 2000-125506 discloses a spindle motor that adopts a pair of upper and lower ball bearings 71 and 72 instead of a hydrodynamic bearing as shown in FIG. 12. According to this, a screw hole 75 for fixing a clamp 74 at an upper end portion of a shaft 73 is formed. When a wall thickness in the diameter direction between the outer peripheral surface of the shaft 73 and the screw hole 75 is set as t, and a root diameter of the screw hole 75 is set as D0, the dimension is set so as to satisfy the following expression.t≧D0/2  expression (1)
By satisfying the expression (1), an inner ring of the upper ball bearing 71 can be prevented from being deformed by the stress at the time of fastening a screw 76 into the screw hole 75.
However, in the spindle motor shown in FIG. 12, there is the problem that an outside diameter of the shaft 73 becomes large (increases) when the expression (1) is to be satisfied, which becomes a hindrance to miniaturization of the spindle motor.
The present invention has an object to provide a hydrodynamic bearing device capable of preventing an adverse effect by deformation of a shaft when a screw is fastened to a screw hole and promoted in miniaturization, and a compact motor including the hydrodynamic bearing device.